This invention relates to a combination lens system including a basic objective and a rear attachment capable of varying the focal length of the basic objective, and more particularly to an attachment lens having a decreased refractive power to improve image aberrations and particularly field curvature.
It is known to provide a front or rear attachment lens for camera objectives of fixed focal length to increase or decrease the focal length of the objective. The front attachment lens has, however, a disadvantage of requiring a large size. The rear attachment though having an advantage of requiring a smaller size than that of the front attachment has the inherent difficulty that it is hard to achieve good aberration correction and particularly field curvature. The presently available attachments of the latter type are generally characterized by the lack of sufficient lens performance.
A method of improving the above mentioned defect of the attachment lens for Gauss type objectives has been proposed by the present applicant in Japanese Patent Application No. Sho 47-30311 (Japanese Laid-Open Patent Application No. Sho 48-97528). According to this proposal, the attachment lens is divided into two major groups of elements, the front group having a negative refractive power and the rear group having a positive refractive power, so that the front principal point is shifted ahead. Each of the groups is composed of positive and negative lens elements of strong refractive power, the positive lens element being made from low refractive index glass and the negative lens element being made from high refractive index glass so that the Petzval sum is improved.